To reset database to its initial state : https://wordpress.org/plugins/wordpress-database-reset/

Useful when you have installed many themes and wp_options table has got too busy. Or you have installed demo data and now want to wipe it out easily.

When you change thumbnail sizes AFTER you have images uploaded to wordpress, https://wordpress.org/plugins/regenerate-thumbnails/ this plugin will recrop your pre-loaded images based on your new thumbnail sizes.